LATE OLIGOCENE TO EARLY MIOCENE WESTERN CASCADE VOLCANISM IN THE NORTHWESTERN PORTION OF THE CASCADE-SISKIYOU NATIONAL MONUMENT, SOUTHWEST OREGON

The geology of the NW portion of the Monument is defined by upper Oligocene basaltic andesite volcanism that was affected by structural founding during episodic caldera collapse of a younger silicic eruptive complex. The southern portion of the previous recognized Eagle Butte silicic eruptive complex (EBSEC) began erupting during earliest Miocene with rhyolitic to dacitic tuffs (23.53 Ma) and dacitic lava flows (22.8 Ma). These silicic rocks were subsequently affected by caldera collapse that involved pre-existing basaltic andesite volcanoes, the largest of which is the Grizzly Mountain volcano (24.16 Ma). Silicic intrusive rocks, some bearing mercury compounds, were injected along the southwest side of the collapse. Also affected by the collapse were tuffaceous rocks of the EBSEC and initial erosional debris that interfingered with lava and volcaniclastics from concomitant basaltic andesite volcanoes to the south. A series of younger 21.43 Ma rhyolitic to dacitic ash falls and welded ash flows associated with small intrusive centers were emplaced during the last eruptive phase. Many of the tuffs from the EBSEC can be traced to the south where they thin and interfinger with basaltic andesite flows and related volcaniclastic rocks. Lower Miocene (20.2 Ma) basaltic andesite lavas and breccia unconformably overlie the eroded remnants of the EBSEC and interfingering intermediate composition volcanic rocks.